INCREASING THE PLASTICITY OF HYPOEUTECTIC SILUMIN DUE TO DEFORMATION-HEAT TREATMENT
Purpose. To develop a method of deformation-heat treatment of hypoeutectic alloys of the Al-Si system, which makes it possible to grind and spheroidize silicon inclusions, significantly increasing the plasticity of the material and its ability to harden during subsequent cold deformation.
Methods. Manufacturing of castings from Al-Si alloy (7 % Si) in laboratory conditions: casting into an earth mold at a melt temperature of 700 – 720 °С. The castings were subjected to deformation by forging – drawing. Investigation of the microstructure in the cast, annealed and deformed states. Determination of mechanical properties: measurement of hardness, upsetting test.
Results. A method has been developed of deformation-heat treatment of Al-Si alloys (hypoeutectic silumins), which consists of a series of small hot deformations (the beginning of deformation at 510 – 540 °C) with intermediate annealing at temperatures above 500 °C for 10 – 20 minutes each. The method allows one to obtain the degree of deformation of more than 50 % for this class of alloys, known with their low ductility in the cast state. In the course of this deformation-heat treatment in the structure of alloys of the Al-Si system, there is a significant refinement of silicon inclusions and their spheroidization, which increases with an increase in the degree of deformation, and the proposed method also allows more uniform mixing of silicon inclusions in the metal matrix. Both conventional and cyclic (with alternating heating and cooling) annealing does not allow obtaining such a degree of simultaneous grinding and spheroidization of silicon inclusions, which shows the important role of deformation in the developed process of deformation-heat treatment, which allows not only to a greater extent to spheroidize Si inclusions, but and grind them up. It is shown that the developed mode of deformation-heat treatment of alloys of the Al-Si system makes it possible to significantly increase their plasticity, to a much greater extent than annealing. The hardness of the specimens after deformation-heat treatment decreases in comparison with the cast state to the level of indicators of annealed casting. At the same time, the total degree of deformation during deformation-heat treatment had practically no effect on the total hardness (although the microhardness of the metal matrix increased somewhat). The hardness of the material can be increased by work hardening during cold plastic deformation. At the same time, since the material after deformation-heat treatment turns out to be more plastic, it has large reserves for strengthening by this method. Thus, although deformation-heat treatment reduces the hardness in comparison with the cast state, due to the increase in plasticity, it creates a reserve for much greater hardening of the material due to cold plastic deformation than can be achieved for a cast or annealed material.
Originality. The dependence of the size and degree of silicon inclusions elongation in the Al-Si eutectic in Al-Si alloys with 7 % Si on the parameters of deformation-heat treatment is determined, and statistical characteristics of the size distribution of inclusions are obtained. The possibility of additional hardening of hypoeutectic silumins after spheroidizing deformation-heat treatment due to cold plastic deformation is shown, since an increased plasticity margin allows one to obtain greater degrees of deformation in the cold state than is possible for a cast or annealed material.
Practical implications. The possibility of creating ductile alloys of the Al-Si system (silumins) by deformation-heat treatment is shown, as well as the possibility of varying, if necessary, the hardness and plasticity properties in a much wider range than in the as-cast state.
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